Traditionally, inorganic silicon and gallium arsenide semiconductors, silicon dioxide insulators, and metals such as aluminum and copper have dominated the semiconductor industry. In recent years, however, there has been an increasing research effort in using organic thin-film transistors (OTFTs) as an alternative to the traditional devices based on inorganic material sets. Among other benefits, the use of organic materials may enable lower cost manufacturing of electronic devices, large area applications, and the use of flexible circuit supports for display backplanes or integrated circuits.
A variety of materials have been considered as organic semiconductors, with the most common being fused acenes such as tetracene and pentacene, oligomeric materials containing thiophene or fluorene units, and polymeric materials like regioregular poly(3-alkylthiophene). While polymers may be coated from solution, device performance is poor when compared to well-ordered thin films prepared by high vacuum vapor deposition. Positive charge-carrier mobility (p-type) as high as 3.3 cm2 V−1 s−1 (Kelley, T. W.; Boardman, L. D.; Dunbar, T. D.; Muyres, D. V.; Pellerite, M.; Smith, T. P. J. Phys. Chem. B 2003, 107, 5877–5881), on/off current ratios greater than 108 (Knipp, D.; Street, R. A.; Völkel, A.; Ho, J. J. Appl. Phys. 2003, 93, 347–355), and sub-threshold voltages of less than 0.5 V (Klauk, H.; Halik, M.; Zschieschang, U.; Schmid, G.; Radlik, W.; Weber, W. J. Appl. Phys. 2002, 92, 5259–5263), have been reported for pentacene-based transistors. These values are comparable or superior to those of amorphous silicon-based devices.
However, there are several areas where an alternative semiconductor material could offer improvements. The device architecture, choice of materials and substrate roughness all affect device performance. In pentacene-based devices, these variations have, in part, been attributed to the existence of several polymorphs (Mattheus, C. C.; de Wijs, G. A.; de Groot, R. A.; Palstra, T. T. M. J. Am. Chem. Soc. 2003, 125, 6323–6330). The alignment or structural order of the pentacene molecules differs for each polymorph or crystallographic phase, and this structural order determines the electronic properties of the device. The crystallographic phase adopted by pentacene depends on the process and conditions under which the crystals are formed. The thin film form of pentacene can be converted to the bulk phase by exposure to solvents such as isopropanol, acetone or ethanol. (See, for example, Gundlach et al., Appl. Phys. Lett., 2000, 74(22) 3302.) Additionally, the long term oxidative and thermal stability of pentacene is unknown, as is the lifetime of pentacene-based semiconductor devices. The ease of synthesis and purification is another factor that must be considered in regard to the utility of an organic semiconductor. In particular, soluble materials may be purified by recrystallization or chromatography, familiar techniques that are not available for fused acenes like pentacene. The ability to construct devices using solution processing techniques is potentially key for realizing a low cost manufacturing process. And lastly, it is likely that a variety of organic semiconductor materials possessing a range of physical and chemical properties may be required for specific applications.